You won't be able to see anyone else in the room if you are playing multiplayer and you won't be able to see your instrument. I know looking at the toy instrument is rare in Rock Band but I still feel like Rock Band will have its immersion hurt by VR rather than helped.
Drums would be tricky. They'd basically have to replicate the rock band drum layout rather than have various drum set options, because otherwise it would just screw with your head regarding where you need to hit.